Grounding a small 120/24 V control panel step by step

Short answer: land every AC ground on one isolated copper bar, size each green conductor from the breaker ahead of it — 14 AWG copper on a 15 A branch — and bond the bar to the enclosure wall, the backplane and the door with one jumper each. Leave the 24 V 0 V floating on a digital panel, and bond it to the bar at a single point when analog inputs need a reference. The scheme below fits a bench build fed by a cord: a 5 A, 24 V DIN-rail supply, a compact PLC, one ground bar and three stud positions on your cutout drawing.

What does a ground do in a small 120/24 V panel?

The ground in your panel has one job that matters for safety: give a 120 V fault a low-resistance metal path back to its breaker, and on a 15 A branch that means a 14 AWG copper equipment grounding conductor sized per NEC Table 250.122.

NEC 250.4(A)(5) calls that route the effective ground-fault current path and counts the earth out of it. The green conductor in the cord carries the whole fault; the soil under the bench carries none of it. A 24 V circuit runs fine with no earth at all, so the reference side of grounding is about noise, not safety.

Three parts of the panel share the safety job. NEC 250.96(A) calls for metal parts that could carry fault current — enclosure, backplane, door — to be bonded together. NEC 250.6 keeps neutral current off that metal, so the cord's white conductor lands on its own terminal block and the ground bar never sees load current. The long-form rules sit in the grounding and bonding guide.

What ground wire size does each conductor need?

Size every AC equipment grounding conductor from the breaker ahead of it with NEC Table 250.122: 14 AWG copper under a 15 A breaker, 12 AWG under 20 A, 10 AWG under 30 A, 8 AWG under 100 A.

Buy it green: NEC 250.119 puts insulated grounding conductors 6 AWG and smaller in green or green with yellow stripes, so no tape and no guessing at the far end. On the 24 V side, 18 AWG carries signal commons and shield drains; when 16 AWG earns its place is the subject of the 16 vs 18 AWG control wiring article. A conductor carrying load return sizes as a power wire, so this example's 5 A return uses the same copper size as the EGC.

The supply arithmetic, on example numbers: 24 V × 5 A = 120 W out; at an assumed 88% efficiency that is 120 ÷ 0.88 = 136 W in, which is 136 ÷ 120 = 1.1 A on the 15 A branch — spare capacity before the breaker cares. The branch breaker protects the wire, so it stays at 15 A while the supply gets its own device, a 6 A curve C breaker in this example; the supply's manual names its own value.

Do you need an isolated ground bar or grounding terminal blocks?

Use an isolated copper bar for this build: a bar that takes #14 to #2 AWG lugs on seven or more circuits, mounted on insulating standoffs, so its only path to the panel runs through your single bonding jumper.

The standoffs buy you one defined meeting point. Analog grounds, fault current and shield drains all arrive at the bar by the conductor you chose, instead of finding routes through rail feet, duct brackets and mounting screws. Where the bar mounts, wall or floor, gets its own answer in the ground bar placement article, and the case against leaning on the backplane as a ground plane is made in the backplane bonding article.

Grounding terminal blocks work by gripping the DIN rail: the block's foot is made to bridge conductor to rail, so blocks on one rail stay connected without jumpers. A digital panel with a handful of grounds can lean on that. A test or analog bench treats the rail joint as one more invisible path, so either jumper the blocks along the rail and land a rail end on the bar, or drop the blocks and fit the bar.

Decision: digital-only and simple, jumpered grounding blocks; test bench or analog in the panel, the isolated copper bar.

How do you bond the bar to the enclosure, backplane and door?

Run three green jumpers of the same copper size as the EGC with crimped ring lugs: bar to enclosure wall stud, bar to backplane stud, enclosure stud to a door bolt dedicated to grounding, each under a toothed washer on bare metal.

NEC 250.12 takes paint, lacquer and enamel off threads and contact surfaces at grounding connections unless the fitting is designed to bite through them. A stainless wall is already bare, so the washer has clean metal waiting; a powder-coated wall gets a bright ring filed around the stud first. Fourteen AWG copper runs about 2.5 Ω per 1000 ft (8.3 Ω per km), so a 1 ft [300 mm] jumper adds roughly 0.0025 Ω — a meter finds joint resistance, not wire resistance.

The door bolt exists for the jumper alone; a hinge swings, corrodes and was never a grounding part. Jumper size, lug choice and the continuity test for a door are worked through in the door bonding article. A build that steps its bonds to 8 AWG on a 100 A feed deserves a matched crimp die — page on selecting crimp tools for #6 and #8 AWG copper conductors covers die and tool choice.

Where does the 24 V 0 V side land?

Leave the supply's 0 V floating on a digital-only panel — the PLC runs fine with no ground reference — and bond the DC common to the bar at a single point when analog inputs or long signal runs need one.

A supply with no DC ground terminal is floating by design; its common sits wherever leakage puts it, usually within a volt or two of earth in a clean panel. One bond gives analog circuits a defined reference and gives picked-up RF a way out. Two bonds make a loop, and the duct and rail become its antenna. Signal shields land on the bar with 18 AWG drains, one end only, so the cable's noise reference stays at your end. The bond-or-isolate case for AC and DC grounds sharing one enclosure gets the full treatment in the AC and DC grounds article.

In what order do you wire the grounds?

Wire the grounds before any power conductors: cord EGC to the bar first, then the three bonding jumpers, then each device ground, then the DC common decision last, so every lug lands on a bar already bolted to the enclosure.

1. Bolt the isolated copper bar to the enclosure floor on its standoffs, out in the open where ducts will never hide it.
2. Bring the 14/3 SJT cord through a cord grip and land its green conductor on the bar; the connector and bond at the entry wall are covered in the grounding conductor entry article.
3. Jumper the bar to the enclosure wall stud, ring lugs both ends, toothed washer under each lug.
4. Jumper the bar to the backplane stud the same way.
5. Jumper the enclosure stud to the door bolt, then mark all three studs with green ground labels.
6. Land the supply's PE terminal on the bar with its own conductor.
7. Land the PLC's frame ground and every other device ground on the bar, one conductor per device.
8. Land the cord's neutral on its own terminal block, away from the bar.
9. Feed the supply through its own breaker — a 6 A curve C device in this example; the supply's manual names its value.
10. Decide the DC common last: floating on a digital panel, or one bond to the bar for analog.

What tests prove the ground path?

Power off, then measure 0.1 Ω or less from the bar to the door, the backplane and the enclosure wall with a 4-wire milliohm meter, door closed and latched.

Lock the panel out and confirm zero voltage at the line terminals before the first clip goes on. A 2-wire meter adds 0.1 to 0.3 Ω of its own lead resistance, so zero the leads or subtract them first. Wiggle each jumper while watching the reading; a number that jumps is a joint that fails within the year. With the supply and PLC disconnected, a 500 V insulation meter from line and neutral together to the bar reads in the hundreds of megohms on healthy new wiring.

The fault math makes the point. A 30 ft branch run out and back is 60 ft of wire, or 60 × 0.0025 = 0.15 Ω, so a dead short from line to the bonded wall pulls about 120 ÷ 0.15 = 800 A — more than 50× the 15 A rating and straight into the breaker's magnetic trip. Copper wins this race for that reason alone: 0.15 Ω of branch plus 0.1 Ω of tired joints still trips, but an unbonded powder-coated wall could sit at line potential until a hand finds it.

What goes on the enclosure order?

Order the box with three stud positions on the cutout drawing — enclosure wall, backplane and a dedicated door bolt — in 304 stainless for a bench or washdown site, delivered in 10 business days from your approval of the drawing.

Stainless arrives bare, so every ground contact is metal on metal with nothing to file; 316L is the pick in salt air, brine or chemical areas. The enclosure builder takes the layout step by step, and the custom enclosure page covers material and size choices from one piece. Our stainless boxes are rated NEMA 4X / IP66 by the manufacturer; they are not UL listed. Every hole on the drawing decides which rating survives the build — what a panel keeps after you cut it and mount devices works that out. If the finished panel ships on a machine, NEC 409.2's industrial control panel definition and a UL 508A listing enter the picture, and that conversation belongs to the panel shop.

The decision, both ways: a digital bench panel takes jumpered grounding terminal blocks and a floating DC common; a test or analog panel takes the isolated copper bar, three bond jumpers and a single common bond. Both schemes end at the same number — every bonded part reads under 0.1 Ω to the bar, door closed — and both start at the same place: one bar, one point, one path per conductor.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

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